The present invention relates to an optical deflector in an optical scanning unit for use on laser printers, digital copiers, laser facsimiles, POS terminals or the like.
FIG. 5 shows a typical construction of the optical scanning unit for which the optical deflector of the present invention is to be used. A light beam emitting from a light source such as a semiconductor laser (not shown) arrives at a polygonal mirror 3 in the optical deflector 1. The polygonal mirror 3 which is rotated by a motor 2 deflects the incident beam to produce a scanning beam B. The beam B passes through an imaging lens group 5 for producing the right image on a subject 4 to be scanned. The beam further passes through a dustproof glass window 6 to reach the subject 4, thereby forming a latent electrostatic image in accordance with the electrophotographic process or performing exposure on a film which is popular at present. The motor 2 is typically a DC brushless motor and screwed to an optical box 7. A cover 8 is provided to remove dust in the whole of the unit.
The conventional optical deflector typically uses a coreless motor which does not have a core in the stator. The construction and operation of the conventional optical deflector are described below with reference to FIGS. 6(A) to 6(C) and FIG. 8.
Each of FIGS. 6(A) to 6(C) shows the conventional optical deflector that uses a dynamic pressure spindle as a bearing. As shown in FIG. 6(A), a stationary shaft 10 is secured to the motor body 12 by means of a screw 13. The stationary shaft 10 is made of either stainless steel or plastic material. An outside diameter of the stationary shaft 10 is manufactured to a precision on the sub-micron order. An annular stator yoke 14 which is made of either a ferrous metal or ferrite is secured to the motor body 12 by means of a screw or adhesive. Provided on top of the stator yoke 14 is a stator coil 16 which is secured to a stator coil base 15.
A tubular rotating shaft 11 is fitted over the stationary shaft 10. A flange 17 is formed around the rotating shaft 11 as an integral part. The underside of the flange 17 is worked to form an annular reference face 17a that serves to provide the necessary horizontal precision for the rotating faces of a polygonal mirror 3. The polygonal mirror 3 and a magnet 19 are fitted over the rotating shaft 11 in a position that is below the flange 17. Thus, the rotating shaft 11, the polygonal mirror 3 and the magnet 19 combine to form a rotating element 18. A groove 11a is formed in the lower part of the rotating shaft 11 and a leaf spring 20 is placed between the groove 11a and the magnet 19 to establish elastic engagement. As shown in FIG. 6(B), the leaf spring 20 has an annular shape, with a plurality of engagement lugs 20a and 20b being formed on the inner and outer circumferences, respectively. The inner lugs 20a is engaged with the groove 11a as shown in FIG. 6(C). The outer lugs 20b are elastically urged against the bottom of the magnet 19. The polygonal mirror 3 is fixed in position by the leaf spring 20 which depresses the mirror toward the reference face 17a of the flange 17, with the magnet 19 being interposed.
The outer circumference of the stationary shaft 10 is provided with grooves 10a that are etched in a herringbone pattern. The inner circumference of the rotating shaft 11 is planished by boring to a surface precision on the sub-micron order that is as high as the outside diameter of the stationary shaft 10. The rotation of the shaft 11 combines with the herringbone grooves 10a on the stationary shaft 10 to create a pneumatic dynamic pressure that is sufficient to support the rotating shaft 11 in a radial direction (vertical direction with respect to the rotating center axis). The top of the stationary shaft 10 is provided with a magnet 21 that repels a magnet 22 on the ceiling of the rotating shaft 11 to support the latter in the thrust direction (along the rotating center axis).
FIG. 7 is a graph showing the distribution of dynamic pressure in the direction of the rotating center axis. Symbols K1 and K2 in FIG. 7 denote the rigidity of a bearing and they provide pressure P1 and P2, respectively. This is equivalent to saying that ball bearings are present in the areas where P1 and P2 are produced. These ball bearings constitute a dynamic pressure bearing system.
The dynamic pressure bearing system in the optical deflector described above is of such a type that the stationary shaft 10 is provided on the motor body 12, with the hollow rotating shaft 11 being fitted over the stationary shaft 10. In a second type of optical deflector, it is known to provide a hollow stationary shaft on the motor body, with a rotating shaft being inserted into the stationary one. However, motor design requirements and the need to insure the reliability of the dynamic pressure bearing system have practically ousted this second type from industry and the first type illustrated in FIG. 6(A) is most commonly used today.
Another conventional optical deflector that uses ball bearings is described below with reference to FIG. 8. The components or parts that are the same as those corresponding to the optical deflector shown in FIG. 6(A) are identified by like numerals and not described in detail.
Two ball bearings 23 and 24 are fitted in the space between the stationary shaft 10 and the rotating shaft 11. The top of the rotating shaft 11 is closed with a seal 29 in the form of a high-polymer sheet or a metal plate that is bonded with an adhesive or a tackifier to prevent grease from splashing up out of the shaft 11 as the ball bearings 23 and 24 continue to rotate. The lower part of the rotating shaft 11 is provided with labyrinth grooves 30 in the space between the motor body 12 and the rotating shaft 11. The rotation of the shaft 11 causes air to be alternately compressed and rarefied to form an air curtain that prevents grease from splashing down out of the shaft 11 as the ball bearings 23 and 24 continue to rotate.
The conventional designs shown in FIGS. 6(A) to 6(C) and FIG. 8 have the characteristics listed in the following table 1.
TABLE 1 ______________________________________ FIGS. 6(A) Parameter to 6(C) FIG. 8 ______________________________________ Limit of the distance .sup. 70.phi. .sup. 70.phi. between opposing faces of the polygonal mirror (in terms of inscribed circle), mm Limit of the average 10 10 thickness of the polygonal mirror, mm Limit of the outside .sup. 50.phi. .sup. 50.phi. diameter of the magnet, mm Average thickness of the 5 5 magnet, mm Limit of the rotational 25,000 15,000 speed, rpm ______________________________________
A problem common to the designs in FIGS. 6(A) to 6(C) and FIG. 8 is that if they are used at rotational speeds exceeding 10,000 rpm, a very strong "whine" is caused by the polygonal mirror 3. In either design shown in FIG. 6(A) or FIG. 8, this problem is solved by a cover 32 fitted with the light-transparent window glass 31 secured to the motor body 12 by a suitable member such as screws (not shown). The cover 32 renders the optical deflector both soundproof and dustproof.
As described above, the conventional optical deflector shown in FIG. 8 has the labyrinth grooves 30 formed in the lower part of the rotating shaft 11 so that grease does not splash down as the ball bearings 23 and 24 continue to rotate. However, if the optical deflector is operated in a hot (60.degree. C.) environment at a rotational speed exceeding 10,000 rpm, splashes of grease 33 (also called an "oil mist") are prone to occur extensively in the directions indicated in FIG. 8. Since the optical deflector forms a closed structure, the splashes of grease 33 are inevitably deposited on the polygonal mirror 3 and the window glass 31 and this has caused inconveniences such as a drop in laser power. Even in the absence of such grease deposition on the polygonal mirror 33 or window glass 31, the splashing of grease has led to a marked shortening of the service life of the ball bearings 23 and 24.
The possible cause of this phenomenon is described below with reference to the graph shown in FIG. 8 which depicts the distribution of air pressure due to the rotation of polygonal mirror 3. As it rotates, the polygonal mirror 3, working like blades of a cross-flow fan, aspirates air both from above and below and the resulting laminar flow of air is ejected to produce pressures that peak at the greatest "positive" value shown in the graph of the FIG. 8. As the reaction to this formation of "positive" pressures, the pressure in the space between the stator coil base 15 and the magnet 19 indicates a very great "negative" value. The pressure performing this pumping operation increases with the rotational speed and outside diameter of the polygonal mirror 3 but decreases with the number of its faces. The pressure in the space between the magnet 19 and the stator coil base 15 is affected by the pumping operation, and indicates a very great "negative" value. This negative pressure, as it combines with the drop in the viscosity of grease at elevated temperature, causes the grease in the lower bearing 24 to overcome the holding effect of the labyrinth grooves 30, with the result that it is easily sucked out of the bearing to cause extensive fouling under the cover 32.
The pumping operation of the polygonal mirror 3 also works in the conventional design shown in FIGS. 6(A) to 6(C) and the dynamic pressure distribution given by the dynamic pressure spindle is deviated from the normal setting indicated by the solid line in FIG. 7 and the actual curve becomes deformed as indicated by the dashed line, producing maximum pressures at P1' and P2'. As a result, the rigidity of the bearing is lowered to cause "galling" on the dynamic pressure spindle.
To prevent the formation of grease splashes 33 in the optical deflector shown in FIG. 8, Unexamined Japanese Patent Publication No. 309066/1991 proposed that a small gap be provided between the rotating shaft and the motor body and that an air steam generating portion as typified by a spiral groove for generating a forced air stream flowing towards the center axis of the rotating element be provided in the wall surface of either the rotating shaft or the motor body. However, when the present inventors conducted an experiment on this optical defector as it is equipped with a larger polygonal mirror (octahedron whose distance between opposing faces is 70 mm.sup..phi.) that is set up in an ambient temperature of 60.degree. C. and rotated at a speed of 12,000 rpm, the splashing of grease occurred very soon. In other words, the rotating larger polygonal mirror worked like a cross-flow fan, which aspirated the surrounding air by a force strong enough to overcome the aforementioned forced air stream flowing towards the rotating center axis. Another factor that must be considered is that the viscosity of the grease decreased in the hot environment and became more prone to splashing.